Method of deresinifying cellulose derived from wood rich in resin



3,055,794 METHOD OF DERESINIFYENG CELLULOSE DE- RIVED FROM W001) RICH INRESIN Olof Bertil Lundn, Harnas, Karl Esbiiirn Eftring, Falun, andBertil Fredrik Bjelke, Skutskar, Sweden, assignors to Stora KopparbergsBergslags Aktieholag, Faluu, Sweden, a corporation of Sweden No Drawing.Filed Nov. 30, 1956, Ser. No. 625,233 Claims priority, applicationSweden Dec. 1, 1955 6 Claims. (Cl. 162-86) In the manufacture ofcellulose from wood rich in resin, such as pine, it is necessary toremove most of the resin before straining the wood pulp, so that thepulp will be capable of being strained and guided on in the paper millwithout resin trouble arising. The amount of resin permissible in thefinished pulp so that no resin trouble will arise depends on a largenumber of factors which manifest themselves to a different degree indifferent paper mills. It has been found in practice that the content ofextractable resin in the pulp should be less than 1%, if a tolerablygood guarantee against resin troubles is to be obtained.

When digesting hearty pine wood i.e., pine wood having a high content ofheart wood, using, e.g., the sodium bisulphite method, the cooked pulp,after removal of the spent sulphite liquor in the ordinary way, contains2 to 5% of resin. It has been suggested before now to treat such pulpwith sodium hydroxide lye to as to remove the resin. Thus NorwegianPatent No. 56,432 describes a method in which the pulp is treated, atleast pulp concentration, with NaOH under vigorous pounding for thepurpose of defibrating the fibres to such extent that the resin can besqueezed out and made accessible to the dissolving influence of thesodium hydroxide lye.

Renewed, detailed experiments have now shown that it is possible toreduce the resin content in sulphite pine pulp rich in resin appreciablybelow one percent in a simpler way and one less detrimental to thefibres, viz. without appreciable mechanical action on the fibre mass, ifthe pulp is produced according to the bisulphite method using a baseforming soluble soaps with resinous and fatty acids.

In one series of experiments a pulp produced by cooking with sodiumbisulphite in two stages and released from sulphite lye in a diffuserwas treated at 3% pulp concentration and 20 C. under gentle stirringwith varying amounts of sodium hydroxide lye for diiferent periods. Thepulp Was then exposed to a weak vacuum on a vacuum filter bed and washedwith water.

The resin content of the mass (in percent resin calculated per 90% pulp)was found as- Untreated pulp: 3.6% resin Alkali-treated pulp:

It is thus possible, with a few minutes simple stirring at low pulpconcentration, to get down to resin contents below 1%, by using between1 and 3% NaOH, calcu lated on the basis of the dry pulp. Repeated testshave shown that in certain cases less than 1% is sufiicient forobtaining resin contents of about 1%.

Good results have also been obtained with, e.g., soda and other alkalinesalt solutions, pH-values between 9.0 and 12 having been found to beparticularly suitable. Acidification may be carried out immediatelyafter resin extraction, but a somewhat lower resin content is often3,fi55,794 Patented Sept. 25, 1962 ice obtained, if the pulp ispreviously concentrated and/or pressed out.

It is seen that the resin can in this way, by gentle treatment at lowpulp concentration and low temperature, be extracted from the fibreswithout extraction being preceded by mechanical action according toNorwegian Patent No. 56,432. It can be shown by microscopicinvestigation that the resin is, to a large extent, dissolved out of themeristem fibres also, and that it is therefore not necessary, in orderto reduce the resin content, to remove these by the so-called Schibbyprocess wherein short fibres containing resinous materials are screenedout, or the like. It has thus proved possible to deresinify the pulpwithout any stirring at all, eg in a diffuser, in connection with theremoval of lye and washing of the pulp. The same good results are alsoobtained with continuous counter-current washing, 'e.g. on rotatingfilters or plane filters. By mixing the washing liquid, at a convenientstage of the washing cycle, with a quantity of sodium hydroxide lyecorresponding, e.g., to 1 to 3% of the quantity of pulp, it is possibleimmediately to obtain a pulp containing less than 0.5% of resin. Thiscan be done without appreciably lengthening the duration of the washingcycle. It is thus a distinctive feature of the invention that the pulp,if desired, can be deresinified without stirring by treatment withalkaline solutions of, e.g., sulphite lye and/or water.

At which stage in the treatment the alkali should be added depends onthe form in which it is desired to obtain the resin. In certain cases itmay be useful first to wash with dilute lye from a previous cookingstep, then with alkalised dilute lye of said kind and/or water andfinally to wash with water. In this way the resin solutions are obtainedseparate from concentrated or diluted lyes, respectively.

It has been found possible to carry out deresinification of the pulp bysupplying, to a diffuser charged with pulp and spent liquor, alkalisedsolutions of spent sulphite liquor (possibly strongly resinous) and/oralkalised water. During passage through the acid pulp the alkali iswholly or partly absorbed, the pH of the solution sinking in theprocess. In case the solution supplied contains soluble resin soapsand/or fatty acid soaps, these are precipitated in finely dispersed formas acids, which will not, however, fasten on the pulp but will becarried along by the passing solution. Acids dissolved out of the alkaliwhen the solution enters the pulp behave in the same way. It is seenthat in this way free, suspended organic acids can during the washingprocess be transferred to the strong liquor, and their heat ofcombustion can be utilised while they are burnt (in a series of suchtests 6075% of the resin content of the mass could be transferred to thestrong liquor). Towards the end of the washing process the pH-value hasrisen to such an extent that the acids are kept in solution. Thesesolutions may advantageously be used in the next washing process in thediffuser.

By taking care that suitable concentration and temperature conditionsprevail in different solutions it is possible to achieve that a layer ofstrongly alkaline solution slowly sinks down through the diffuser,preceded by, e.g., acid strong liquor and followed by solution withrelatively low but still alkaline pH-value.

It has further turned out that unsaponifiable and thus in themselvesinsoluble resinous materials are, during the dissolution of resinous andfatty acids, dispersed and removed from the pulp to a considerableextent. The resin content of the solution removed, eg the strong liquor,gets particularly high if base addition is in the main carried out atthe start of the washing process with alkaline solution. Hence anadvantage of the washing quired for deresinification can be transferredto the strong waste liquor and recovered after this has been burnt. Byburning the resinous material simultaneously with the dry portion of thewaste liquor their heat of combustion is therefore utilised and it is,moreover, achieved that pollution of water courses by them is preventedor substantially reduced.

Strong liquors containing up -to 1% or more of resin can be fermented toyield alcohol etc. in the usual way.

Under certain circumstances it is advantageous to alkalise the solutionswith a buffer salt, e.g. soda and, besides, possibly NaOI-I. By this itis achieved that the pH-value of the supplied solution can be keptlower, which may be favourable for the colour of the pulp. It has turnedout to be particularly useful to start the washing process with a highpH-value, which should, however, preferably not exceed about 12 and morepreferably not exceed 11.2, and to use a lower pH-value for thefollowing washing operation, e.g. 9-11.

Certain precautions have to be observed in carrying out such aderesinification. Thus it is true that the temperature mayadvantageously be above 20 C., though preferably not above 60 C. for anextended period, but it has turned out that a combination of raisedtemperature and raised alkali concentration can produce discolourationof the unbleached, lignin-containing pulp. This discolouration, it istrue, may for the most part disappear during subsequent washing andacidification, if any, but it may unnecessarily give the finished pulp asomewhat reduced whiteness compared with what is normal.

It is useful, for instance when emptying the diffuser, when the pulp isusually diluted with water, to mix it with some acid (S or H 80 so as toprevent precipitation and staining of lime soaps. Favourable resultshave also been obtained by acidification of the water at the end of thewashing process.

It is also useful to start adding sodium hydroxide liquor while the pulpis slightly acid and before it has been treated, at a relatively highpH-value (e.g. above 4) with larger amounts of calcium-containing water,for the Ca ions may combine with resinous and fatty acids and rendertheir release in a subsequent alkaline treatment more diflicult.

If a very low resin content is desired, e.g. down to about 0.1 to 0.2%,it is useful to suspend the pulp deresinified as above and in suspensiongive it an after-treatment with small quantities of base. If desired,the mass may subsequently be acidified.

In the example below is shown a Way of carrying the invention intoeffect. 7

In a diffuser (blowing tank) containing pine pulp of 16 tons dry weightmade soluble by means of sodium bisulphite and for the rest filled withsulphite liquor of a dry weight content of 17% (strong liquor), washingwas started with medium liquor from a preceding diffuser washingoperation of the same kind. This medium liquor, which had a dry weightcontent of 9.5%, contained resin (dissolved from the preceding cook) inthe form of a suspension and as-a solution of alkaline soaps, andbesides 300 kg. added sodium carbonate. This solution was fed to thediffuser at a speed of 40 m. /h. Hereupon the pulp was washed withdilute liquor of a dry weight content of 4%, also obtained from apreceding washing operation of the same kind and with a pH-value ofabout 9 and holding substantial amounts 4 of resin in suspension. Thepulp was subsequently washed clean with water.

The un-washed pulp contained 4.1% of resin (methylene chloride extract),i.e. 650 kg. After the mass had been washed in the way described above,it contained 0.65% of resin and, after having been strained and passedon to the dryer, 0.55%, i.e. kg. of resin. In the strong liquor were tobe found 420 kg., i.e. 65% of the original resin content of the pulp,whereas it follows that only kg. of resin had run off in the washingwater or the like.

The strong liquor was cooked down and burnt, after which the 300 kg.soda used for washing were found accessible in the melt from the ovenand could be recovered the form of sodium bisulphite to be used in thenext cook. Combustion of 420 kg. of resinous material produces about 4.5tOns of steam, i.e. 280 kg. of steam per ton pulp mass.

What we claim is:

1. A method of producing cellulosic pulp having a low resin content fromwood rich in resin comprising preparing delignified pulp by completelycooking said wood with a sulphite cooking liquor containing a base whichforms a soluble soap with resinous and fatty acids, and then washing thedelignified pulp containing sulphite cooking liquor with an aqueousalkaline solution having a pH between 9 and 12 at a temperature betweenabout 20 C. and 60 C. without substantial agitation for a period of upto about one hour and prior to any other washing step to remove resinswithout further modification of the pulp and then further washing withwater the quantity of alkali in said solution amounting to from 1 to 3%by weight of the pulp, on a dry basis.

2. The method of claim 1 wherein said pulp is washed in a diffuser witha mixture of the alkaline solution and sulphite liquor from a precedingwashing operation to which additional aqueous alkaline solution has beenadded followed by said further washing with clean water.

3. The method of claim 1 wherein the aqueous alkaline solution comprisessodium hydroxide.

4. A method as claimed in claim 1 wherein said a1- kaline solution isbuffered with soda.

5. A method as claimed in claim 1 wherein said treatment is carried outat 20 C., with sodium hydroxide in the absence of any substantialagitation of said pulp using a pulp concentration of 3% and a quantityof sodium hydroxide amounting to 1 to 3% of the quantity of dry pulp.

6. A method as claimed in claim 1 wherein said pulp is pine pulp.

References Cited in the file of this patent UNITED STATES PATENTS1,640,853 Richter Aug. 30, 1927 1,742,219 Richter Jan. 7, 1930 1,841,551Richter Jan. 19, 1932 1,880,044 Richter Sept. 27, 1932 1,899,637 RichterFeb. 28, 1933 2,385,955 Tomlinson Oct. 2, 1945 2,508,043 Schaefer May16, 1950 2,528,350 Farber Oct. 31, 1950 2,542,801 Dela Roza Feb. 20,1951 FOREIGN PATENTS 16,092/28 Australia 596,164 Great Britain Dec. 30,1947 458,897 Canada Aug. 16, 194

1. A METHOD OF PRODUCING CELLULOSIC PULP HAVING A LOW RESIN CONTENT FROMWOOD RICH IN RESIN COMPRISING PREPARING DELIGNIFIED PULP BY COMPLETELYCOOKING SAID WOOD WITH A SULPHITE COOKING LIQUOR CONTAINING A BASE WHICHFORMS A SOLUBLE SOAP WITH RESINOUS AND FATTY ACIDS, AND THEN WASHING THEDELINGIFIED PULP CONTAINING SULPHITE COOKING LIQUOR WITH AN AQUEOUSALKALINE SOLUTION HAVING A PH BETWEEN 9 AND 12 AT A TEMPERATURE BETWEENABOUT 20 C. AND 60*C. WITHOUT SUBSTANTIAL AGITATION FOR A PERIOD OF UPTO ABOUT ONE HOUR AND PRIOR TO ANY OTHER WASHING STEP TO REMOVE RESINSWITHOUT FURTHER MODIFICATION OF THE PULP AND THEN FURTHER WASHING WITHWATER THE QUANTITY OF ALKALI IN SAID SOLUTION AMOUNTING TO FROM 1 TO 3%BY WEIGHT OF THE PULP, ON A DRY BASI